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I believe the Fort Hood shooter used SS198LF in his attack.
He killed 13 and wounded 30 with a 5.7 pistol, including stopping one responding LE with a thigh and knee hit before being taken down by a second LE.
So you can't say it was only effective as an ambush weapon, he took on two 9mm armed LE in a "gunfight" that basically ended in a bit of a draw with none of the three killed.
Is the 5.7 an optimal self-defense cartridge... no.
Will it kill/stop people... yes.
Do some of its positives (like basically zero recoil) outweigh its relatively poor ballistics... I think so for semi-trained people especially.

The SS198LF has a slight bump in velocity over the SS195LF.
Keep in mind I'm getting those velocities out of the "civilian" full length 16" barrel and I bet Doc is basing his "no-go" on the 10" LE/Mil barrel or the 5" pistol barrel.
I know out of the pistol the ammo loses 500fps velocity and only penetrates about 6"-8" (tumbles around 3"), that's not deep enough.
I would guess the LE/Mil P90 penetrates somewhere around 10" which is still a few inches short.

Ah, I see. I was curious if it was possible to make an expanding and less velocity dependent round in something in the size range of the 5.7(i.e. something that could be put in a practically sized pistol, not an AR15 'pistol' sized platform) in line with things like the TSX and Gold Dot loadings in .223 that work better out of short barrels that don't require full length barrel velocities for good performance. Or at least something that approaches decent 9mm performance out of the FiveSeven in terms of consistent deep penetration with some expansion for tissue crushing damage like in most pistol caliber offerings, but retaining the light recoil and control you get with the smaller calibers.
Would something like a 40 grain Gold Dot soft point designed to expand at the velocities obtainable from a 5 inch pistol be able to get the required 12"-18" inches of penetration while say expanding to .30? Or is that basically impossible to accomplish?

Would something like a 40 grain Gold Dot soft point designed to expand at the velocities obtainable from a 5 inch pistol be able to get the required 12"-18" inches of penetration while say expanding to .30? Or is that basically impossible to accomplish?

I'd think you'd run out of available metal or velocity before you're going to get much expansion from a 40gr. .224" diameter bullet.
Since the P90 is a straight blowback design you're not going to be able to up the velocity/pressure without blowing shit up next to your face.
I think the best way to get "expansion" out of these little bullets is a fast yaw and maybe the bullet flattens out to a slightly wider side profile.
Basically what the SS198LF does now on yaw and penetration (except they don't flatten out).

Lot of desert out here.
Lot of holes in the desert.
Lot of problems buried in those holes.

I currently have a PS90 leaned against the wall in my bedroom loaded with 50 rounds of SS198LF (27gr. @ 2500fps). They consistently go around 12"-16" in gel and start tumbling around 5"-6" into the block. That's pretty damn effective, especially with how easy it is to put 3-5 rounds into a target super fast with zero recoil.
It's way easier to shoot under stress than any 9mm pistol for both my wife and my 16 year old boy.
I topped it with an always on Holosun HS503CU and added a Surefire X400U wml/green laser to it.
I have no doubt that's it's a far better "house gun" than pretty much any other option in my extensive collection, by "house gun" I mean a gun that any family member can pick up at any time and be effective with.
Would I be better served with my Benelli M4 and 9 rounds of FC #1B or the Steyr AUG? Probably. Would my wife be able to lay down hate with either one of those as well as with the P90? Hell no.

You've given something to think about. I'm trying to deal with the lack of a good HD long gun. My wife and both my kids are left-eye dominant and are in the habit of shooting long guns left handed (I'm not). My wife hates the AR (thank you Army) and a shotgun is a little more than my youngest daughter is interested in. The PS90 is an option that I hadn't considered. I've found the balance on the 16" actually better than the 10". Though the 10" on full-auto was fun on the test range in Afghanistan so many moons ago.

Originally Posted by Lester Polfus

My wife is a twice a year shooter and that probably will never change. There's plenty of people who will scoff at her 10/22 with a 25 round magazine full of CCI-Mini Mags, but I don't see any of them lining up to let her dump the whole magazine into their chest cavity in three or four seconds.

That's another option. Though in this Commonwealth, I think I'm limited to pre-1994 25 round magazines.

Ancient history but Jeff Cooper once said of a prototype gas operated pistol, that while of no benefit to 9mm, it might make possible a Mach three .17 caliber that would "burn through" any usual body armor.
We don't have the propellants to do that now.

Technical sub-forum, technical content:
OP: The bottom line is that caliber per se (i.e., diameter of projectile) does not determine terminal performance. Note that it seems to be pretty much impossible to load a .380 ACP round that is as effective a defensive round as a 9x19; the .380 ACP is also known as 9x17. A silly two millimeters shorter, and the high-end slugs will not perform.
Can you design a .22 bullet that will perform as well as HST or whatever? Dunno. I have doubts, but I am no expert on the topic.

Digression:

Originally Posted by Jim Watson

Ancient history but Jeff Cooper once said of a prototype gas operated pistol, that while of no benefit to 9mm, it might make possible a Mach three .17 caliber that would "burn through" any usual body armor.
We don't have the propellants to do that now.

Could we profitably revive the .224 Boz?

This reminds me of an email group I was on, the subject of some new wildcat came up, "something" necked down to take "something else". I proposed .50 BMG brass necked down to take .22 or .17 projectiles, and the list owner suggested 20mm necked down for phonograph needles...

Recovering Gun Store Commando. My Blog: The Clue Meter
“It doesn’t matter what the problem is, the solution is always for us to give the government more money and power, while we eat less meat.” Glenn Reynolds

Yes, a common reductio ad absurdism.
The greatest neckdown I know of was the .30 Medical Museum, .30x.50, made to study the effect of shell fragments on cadavers and animals in an aimable platform.
There were also the .22x.300 Weatherby and .22x.284 Win.

The Boz was a real gun, .22x10mm in cahoots with STI and maybe Glock.
The British developers threatened legal action against Colonials making anything of the sort for other than Authorized Personnel.